The basic equatorial sundial is comprised of a gnomon that is aligned with the star Polaris and essentially represents the axis of the earth. The time scale is composed of a semicircle that is aligned with the equatorial plane of the earth. On the time scale, hours of the day are separated by 15 degree increments that correspond with a one hour rotational angle of the earth. Refer to U.S. Pat. No. 2,754,593 (Sunblad), and U.S. Pat. No. 2,192,750 (Mead) which describe an adjustable time scale equatorial sundial.
Equatorial sundials have been described which can measure the date as well as time based on the declination of the sun:
U.S. Pat. No. 89,585 (Johnson) describes a blackened glass sphere with a hole to transmit a ray of sunlight against a two axis grid etched on the opposing surface of the glass. By interpreting alignment of the illuminated dot on one axis a time is determined and by interpreting alignment of the dot on the other axis, an approximate date can be assessed. Unfortunately because of the geometry of a sphere, resolution is only good during midday hours (high solar altitudes), and the observer must follow the grid pattern to extrapolate both date and time.
U.S. Pat. No. 4,102,054 (Lewis) describes an equatorial sundial with a cross hair gnomon (referred to as perpendicular interesting “cords or rods”) that casts a conventional time shadow against an equatorial plane grid. An improvement to approximate date consists of an analemma scale on the approximate bottom half of the longitudinal ring. When the shadow from the intersection of the “cords or rods” passes through the analemma scale, a date can be determined. Unfortunately, this only occurs when the sun reaches maximum altitude (solar noon), and the intersecting shadows pass through the scale (a 30 minute period or less), so the date function has a finite time span of utility. A desirable improvement is a date scale that could be utilized regardless of time of day. This would require an equidistant relationship of the date gnomon and scale regardless of the solar altitude, which changes from sunrise to sunset even though solar declination is consistent.
U.S. Pat. No. 4,135,357 (Ashton) describes a solar chronometer comprised of a semi-cylindrical common time and date scale with a primary polar gnomon that casts a shadow on the x axis of the scale representing time and a secondary equatorial gnomon that casts a shadow on the y axis representing date. From the point where the shadows intersect, the observer must follow the grid to an x and y axis value to approximate the date and the time.
U.S. Pat. No. 4,237,611 (Wurch et al.) describes a shadow scope comprised of a semi-cylindrical scale with a crosshair gnomon that while not a sundial could be used as one. The gnomon casts a crosshair shadow on a two axis grid scale, one axis representing time, the other date. The observer must follow relation of the shadow intersection to an x and y axis value to approximate the date and the time.
U.S. Pat. Nos. 4,785,542 and 4,845,853 (Haskett) describe an equatorial sundial comprised of a point gnomon that casts a shadow dot on opposing semi cylinders, each containing a two axis time and date grid. The primary improvement is intended to be better scale resolution at low solar altitudes. Similar to U.S. Pat. No. 4,237,611, the user must follow relation of the shadow dot to an x and y axis value to approximate the date and the time.
U.S. Pat. No. 4,945,644 (Fuller) describes an equatorial sundial comprised of a gnomon bead casting a shadow on an inverted hemisphere containing a time and date grid. Similar to U.S. Pat. No. 4,237,611, the user must follow relation of the bead shadow to an x and y axis value to approximate the date and the time.
U.S. Pat. No. 5,062,212 (Blaker) describes a cross hair gnomon casting a shadow against a semi-cylindrical equatorial grid containing two axis for date and time. Similar to U.S. Pat. No. 4,237,611, the observer must follow relation of the shadow intersection to an x and y axis value to approximate the date and the time.
U.S. Pat. No. 5,197,199 (Shrader) describes a reflected dot sundial that by an anolimic scale can approximate both time and date. Like U.S. Pat. No. 89,585, this device losses resolution during low solar altitudes (beyond the midday hours) and requires the user to follow a grid pattern to extrapolate both time and date.